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Announced at http://linguistlist.org/issues/18/18-3482.html
AUTHOR: Herschensohn, Julia TITLE: Language Development and Age PUBLISHER: Cambridge University Press YEAR: 2007

Matthew T. Carlson, Department of Psychology, University of Chicago

SUMMARY This book provides a detailed evaluation of the question of a biological critical period (CP) for both first (L1A) and second (L2A) language acquisition, with the primary goal of assessing the existence of such a CP for either of these processes. Given this rather straightforward goal, Herschensohn takes great care to provide a very precise definition of a strictly biological (maturational) CP, and in so doing produces a complex framework for differentiating CP effects from other kinds of age-related changes in the human ability to acquire language. Consistent with this nuanced approach, the book takes a broad perspective on language acquisition that incorporates theories of Universal Grammar (UG), language processing, experience of language, social factors, individual differences, and neural architecture and development. The need for such a complex approach is well-motivated, and these diverse areas of research and theory are well integrated given the constraints of space in this volume. The book is thus well-suited for an audience with some background knowledge in at least some of these areas interested in the effects of age on language acquisition. Readers with particular interest in one area will find that area placed usefully in a broader context. Although the book is couched in terms of a binary question regarding the existence of a biological CP, asking the question separately for L1A and L2A, the result goes beyond simple binary distinctions to open the way for further research on the impact of particular age-related factors on specific facets of language acquisition.

Following an introductory chapter, two chapters each are dedicated to L1A and L2A, first describing the general process of acquisition (Chapters 2 and 4) and then considering the influence of age on each process, respectively (Chapters 3 and 5). The final two chapters amplify the discussion of the linguistic, social, and neurological factors in play and consider the role of age from the varied perspectives presented in the book.

The first chapter lays out the central themes and questions to be addressed in the book, and sets the theoretical context for the discussion. Herschensohn previews the structure of the analyses to come, briefly contrasting the influence of age of acquisition (AoA) on L1A and L2A, introducing the distinction between maturational and experiential factors, and describing how L1A and L2A can be (ethically) investigated. The following section reviews the history of critical period research from the early studies on various nonhuman species, amply discusses the work on birdsong, considered of particular relevance to human language research, and motivates the criteria to be used for judging the evidence regarding language acquisition. These criteria encompass a distinct onset and terminus, relationships to both an intrinsic maturational event and an extrinsic trigger, and effects on a particular system in the organism (pp. 10-12). Crucially, critical periods are seen as a subset of sensitive periods in that critical periods are associated with irreversible changes in brain function (Knudsen 2004), where sensitive periods may be associated with less dramatic changes in ability due to maturation.

After laying this groundwork, the subsequent section on language and the brain reviews the history of critical period research on language since Lenneberg's (1967) influential hypothesis, and sketches out evidence for the localization of language functions in the brain and changes in brain plasticity associated with age, relying mainly on evidence from brain damage. The final section of Chapter 1 frames the discussion in terms of the ''nature-nurture'' debate by outlining 'associationist' and 'modularist' views of human language acquisition, with the former focusing on the role of the environment and the latter on innate predispositions. Herschensohn makes an attempt to find middle ground between these often polarized positions, and the chapter concludes by stressing the combined roles of input and a genetic predisposition, embedding the question of ''which innate and environmental factors determine'' L1A and L2A deeply in the discussion.

Chapters 2 and 4 are structured into sections on phonology, the lexicon, syntax, and morphology, reviewing the sequence and stages of L1A and L2A, respectively. Chapter 2 describes both general stages (e.g. babbling, the optional infinitive period) and acquisitional hierarchies (e.g. uncontractible copula before contractible copula). The discussion is balanced between stressing the evidence for innate structural predispositions and describing the role of environmental factors and cogently, if briefly, assessing a variety of viewpoints on their value, scope, and significance. The review is comprehensive, beginning with the earliest environmental influences on the various components of grammar, from newborns' sensitivity to native language prosody and the development of L1 sound categories. Indeed, the acquisition of prosody before higher levels of structure in L1A will be a central feature distinguishing L1A from L2A. The chapter links the levels of grammar sequentially, from phonology to word learning, and eventually syntax. However, the sequential and dependent nature of different levels of structure and the interdependence between the levels of grammar, such as how the accumulation of lexical items may lead to more detailed phonemic and phonotactic sensitivity (e.g. Vihman 1996), are somewhat simplified.

The third chapter provides a broad and ambitious review of the evidence for age-related declines in L1 acquisition ability, concluding that ''there are periods of heightened sensitivity for the acquisition of first language'' (p. 99) but stopping short of concluding that there is a single critical period. Herschensohn instead argues for a series of thresholds whose decline is not necessarily precipitous, and points out other areas of language (e.g. vocabulary acquisition) in which there seems to be no apparent decline due to age. The chapter consists of two sections, examining exceptional L1A in individuals with atypical brains (e.g. Down Syndrome) and in those with atypical experience of language (e.g. deaf children in certain situations). The first argues for a dissociation between language and other cognitive abilities. The second begins by reviewing cases of extreme isolation, acknowledging the profound limitations of such cases for L1A in general and at the same time making measured claims about what they do reveal. The phenomenon of international adoption is also touched upon, in which children are frequently placed in a different linguistic environment, often after relatively impoverished L1 experiences. Farther on, extensive attention is given to research on deaf individuals whose first access to language may occur at varying ages for various reasons. This population provides the core of Herschensohn's argument for age-related sensitivities to various aspects of language, that contribute to a more gradual decline. Ample attention is also given to creole formation and the notion that children drive the systematization of creoles by imposing their innate linguistic knowledge on more variable pidgin input, although Herschensohn is also careful to point out the controversial nature of this notion of creole genesis (Bickerton 1981, DeGraff 1999).

The fourth chapter parallels the second in describing the process of L2A, similarly addressing the domains of phonology, the lexicon, syntax, and morphology. This chapter sets the stage for the ensuing discussion of age by constructing a nuanced perspective on the comparison of L1 and L2 attainment. Herschensohn is careful to explore the qualitative similarities in both process and grammatical knowledge of L1 and L2, pointing out that differences in final attainment, or even more core UG-related concepts such as parameter clustering, do not necessarily signify a qualitative difference between L1A and L2A. The chapter gives a comprehensive review of older models of SLA, beginning with Contrastive Analysis and spending a fair amount of time on the Fundamental Difference Hypothesis and the ensuing debates about the role of UG in adult L2A. Herschensohn concludes that L1A and L2A are similar, but differ in schedule and in the variety of other factors that come into play. This conclusion of similarity is the pivot into the direct consideration of age in L2A.

Continuing the parallel structure of the book, the fifth chapter broadly examines the question of a critical period for L2A. The chapter is contextualized in a UG framework, and Herschensohn is careful to disentangle the notions of sensitive and critical periods as well as non-biological (non-UG) age-related factors as well. Age-related decline is not the same as a biologically determined sensitive or critical period, and this discussion sets the stage for the subsequent reviews of the literature on AoA, L2 learning in exceptional populations, and end state L2 grammar in children and adults. Based on the ubiquity of variability in L2 attainment regardless of age, the fact that some children acquiring an L2 do not become nativelike speakers, the fact that some adults do become nativelike speakers, and the lack of a clear inflection in age related declines, the chapter concludes that the evidence does not support a critical period for L2A.

Having presented detailed and convincing arguments that age-related changes in L2 acquisition ability are not related to a biological critical period (or at the very least that there is no conclusive evidence for such a view), Chapter 6 moves to consider what other factors might account for the effects of age. Herschensohn first discusses research on external (environmental) and internal (affect, motivation, aptitude, and other learner) variables. The rest of the chapter deals with ''language and the brain'', first giving a basic overview of brain structure and anatomical development, and then discussing a sampling of the psycholinguistic and neuroimaging findings on language processing and localization. The review is necessarily superficial, but supports the conclusion that L2 processing and representation are qualitatively similar to the L1, differing quantitatively in the timing, strength, and extent of neural responses.

The final chapter comprises a detailed summary of the entire book, beginning with a recapitulation of the definition(s) of a biological critical period and the biological and other reasons that might underlie other age-related differences in language acquisition. The bulk of the chapter ties each of the areas addressed earlier in the book to the notion of a biological critical period for either first or second language acquisition, or both. The discussion on language and the brain includes a useful if brief presentation of theories concerning declarative and procedural knowledge (Ullman 2001) and gives welcome importance to the incorporation of processing in theories of bilingualism and L2A. This section is followed by reviews of the conclusions reached for child L1A, child L2A, and adult L2A. While it is necessarily more superficial, readers wishing to gain an overview of the arguments and data could obtain a basic sense from reading this chapter before delving into the greater detail in the preceding chapters. Of course, this also makes it a good recapitulation, and Herschensohn carefully ties each section back to the primary question regarding biological critical periods for language acquisition.

EVALUATION This book is a timely, well-researched, and useful contribution to clarifying the issue of age effects on language acquisition. While Herschensohn is not the first to discuss the specific nature of biological critical periods, this book breaks ground in contextualizing maturational changes among the myriad influences that change how individuals at different ages interact with language in their environment during acquisition. Still more crucially, this book provides a nuanced evaluation of the necessarily different nature of L1A and the learning of any subsequent language at any age, and allows for an understanding of L2A as a continually evolving process responding to biological, contextual, and internal factors but not subject to a critical period per se. The conclusions are a bit more complex for L1A, as Herschensohn distinguishes early periods of heightened sensitivity to some aspects of language, particularly morphosyntax and phonology. The concluding sections state her position perhaps more clearly: ''Language is a human characteristic whose neural expression is established early in childhood, yet it is open to expansion throughout the lifetime in terms of native vocabulary or additional languages'' (p. 240), and after L1A ''the speaker's brain is permanently altered, so acquisition of subsequent languages could never be comparable'' (p. 234), although it clearly builds on the foundation provided by the L1, which accounts for the obvious similarities.

The final paragraph offers a tantalizing new perspective, asking why, in the face of critical periods for many animal communication systems, human language does not show a biological critical period, but can rather be ''reimplement[ed]'' (p. 241). By thus turning the question on its head, Herschensohn opens the door to seeing language acquisition as a lifelong developmental process of interleaved changes in social environment, linguistic input, prior knowledge, processing and learning mechanisms, and even UG. However, the book would benefit from a greater development of this idea and its implications than is present. For example, while knowledge of an L2 is held to be qualitatively similar to L1, the book also sees them as fundamentally different based on the fact that L2A occurs on the foundation of L1A. This leads to the natural conclusion, stated in the book, that L2 speakers can never become completely native in their knowledge, processing, or use of an L2 (although the differences may be exquisitely subtle). Nonetheless, the bulk of the arguments rest on research comparing L2 speakers to native speaker standards. While this is pervasive in the literature and therefore inevitable in a review, the usefulness of the native speaker standard bears some questioning (Birdsong 2005, Hall et al. 2006). More significantly, Herschensohn's perspective invites discussion of the hybridity of multilinguals' competence. Again, this idea of multicompetence (Cook 1991, 2003) is mentioned briefly, but the book stops short of considering this as a primary way of defining the expected outcomes of L2A.

A further minor criticism is the relatively simplified presentation of alternatives to innatism as 'associationist' approaches. There are many diverse ways of modeling the emergence of grammar from experience (Bybee 2006), some of which prefer not to posit any innate structure until absolutely necessary (Langacker 2000), while others attempt to integrate nuanced models with innate prespecifications (Jackendoff 2002), possibly leaving the boundaries of UG for the moment undetermined. While this does not necessarily change the arguments in the book, the portrayal of associationist approaches only in terms of connectionist modeling obscures the detail and empirical scope of work exploring how grammatical categories themselves may arise through domain-general learning processes acting on input, without innate structural specifications. Nonetheless, Herschensohn gives a prominent place to both general and domain-specific processes in driving age effects in language acquisition.

In sum, this book presents a thorough, readable, and well-reasoned discussion of the question of age in both L1A and L2A research. In it, Herschensohn perceptively disentangles the complex facets of the development of the human capacity for language from early childhood through adulthood. She makes sense of a vast body of literature, moving beyond simple questions about the existence of critical or sensitive periods for language to a detailed framework in which the obvious changes in how language acquisition proceeds in different age groups can be profitably examined.

ABOUT THE REVIEWER Dr. Carlson is a postdoctoral fellow in developmental psychology at the University of Chicago. He is interested in probabilistic grammar, the structure of the L1 and L2 lexicon, second language acquisition across age groups, and the role of manual gesture in bilingualism.